Earth

This is Earth, the third planet from the sun. I'm hoping, of all the planets, you're most familiar with this one. Hopefully, though, you're about to
learn a little more about home. When seen from space, one of the most striking features of the Earth is all the water. Our planet's surface is about 70% water.
Earth is the only planet known with liquid water currently on the surface. Mars once had liquid water, and water ice is all over the place, but Earth's water oceans are unique.
The average temperature on earth is about 59 degrees F (15 C). Normally, at earth's distance from the sun, it would be a lot colder, but we have an atmosphere to help us out. Just like Venus' atmosphere traps heat
and keeps the surface blazing hot, our atmosphere keeps the planet nice and warm. You might wonder what the big fuss about the greenhouse effect is,
if it keeps us warm. The big fuss is, that if too much greenhouse gas goes into the atmosphere and traps too much heat, the planet will get too warm and many types of life will start to die. Even a few degrees increase can have major effects.

The earth has a core of solid iron and other heavy materials, surrounded by a liquid iron outer core. The core rotates inside the planet and creates a strong magnetic field. Outside the core is the mantle, a thick layer of molten rock called magma. Floating on the mantle is the crust, a thin layer
of rocks.

The earth's crust is broken into "tectonic plates" which move around. When plates collide, they crumple up and form mountains, like the Himalayas.
When the plates spread apart, magma oozes up from the mantle and hardens into new rocks. Earthquakes are caused when the edges of plates grind together,
and volcanos are often found where two plates meet.

Because the plates are constantly moving around, melting, and reforming, earth's surface is said to
be "geologically active". That's why the earth doesn't have very many craters compared to the moon and most other objects in the solar system. Earth does have some craters though.
One of the most famous is the Barringer meteor crater in Arizona. This crater is almost a mile across and was made by a nickel-iron meteor about 150 feet across. The impact released the same amount of energy as exploding 20,000,000 tons of dynamite!

From the surface, the earth seems like a pretty big place, but in reality, it's a tiny speck. Here is a picture of the earth that was
taken by the Voyager 1 probe in 1990 from a distance of 3.7 billion (3,700,000,000) miles. The earth is the tiny dot. The streaks across the
image are scattered light from pointing the camera so close to the sun. Click the picture to read a quote about it.